Podcast
Questions and Answers
Which of the following is a characteristic of slotted ALOHA?
Which of the following is a characteristic of slotted ALOHA?
In 1-persistent CSMA, a node always waits for the medium to be free before starting to transmit.
In 1-persistent CSMA, a node always waits for the medium to be free before starting to transmit.
True
What is the approximate efficiency of slotted ALOHA?
What is the approximate efficiency of slotted ALOHA?
37%
In CSMA, nodes must ___ before they start transmitting.
In CSMA, nodes must ___ before they start transmitting.
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Match the following CSMA variants with their characteristics:
Match the following CSMA variants with their characteristics:
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What happens during a collision in ALOHA?
What happens during a collision in ALOHA?
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The efficiency of ALOHA is higher than that of slotted ALOHA.
The efficiency of ALOHA is higher than that of slotted ALOHA.
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What does CSMA stand for?
What does CSMA stand for?
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What does RTS stand for in the context of transmission control?
What does RTS stand for in the context of transmission control?
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In IEEE 802.11, a transmission is never successful unless an acknowledgment is received.
In IEEE 802.11, a transmission is never successful unless an acknowledgment is received.
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Name the two types of access methods mentioned in the document.
Name the two types of access methods mentioned in the document.
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The acknowledgment for a successful unicast transmission is sent after a ______.
The acknowledgment for a successful unicast transmission is sent after a ______.
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What happens in IEEE 802.11 if a node wants to send data but detects a busy medium?
What happens in IEEE 802.11 if a node wants to send data but detects a busy medium?
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Match the following terms with their corresponding descriptions:
Match the following terms with their corresponding descriptions:
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What is the role of the base station in infrastructure mode?
What is the role of the base station in infrastructure mode?
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CSMA/CD can operate efficiently in wireless networks.
CSMA/CD can operate efficiently in wireless networks.
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What is a characteristic of CSMA/CD?
What is a characteristic of CSMA/CD?
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ALOHA can be classified as a controlled access method.
ALOHA can be classified as a controlled access method.
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In ___________ Ring, token passing is used to prevent collisions.
In ___________ Ring, token passing is used to prevent collisions.
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Match the following access methods with their descriptions:
Match the following access methods with their descriptions:
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Which of the following protocols is used in IEEE 802.11 WLAN? (Select all that apply)
Which of the following protocols is used in IEEE 802.11 WLAN? (Select all that apply)
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Data frames can vary in size and do not require special boundary detection mechanisms.
Data frames can vary in size and do not require special boundary detection mechanisms.
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Name one security consideration related to data link layer protocols.
Name one security consideration related to data link layer protocols.
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Study Notes
Computer Networking and IT Security (CNS)
- Course: INHH0012
- Semester: WiSe 2024/25
- Instructor: Prof. Dr.-Ing. Stephan Günther
- Institution: Technical University of Munich (TUM)
- Date: Saturday 2nd November, 2024
Chapter 2: Data Link Layer
- Focuses on Local Area Networks (LANs)
- Characterizes connections, multiple access, and access control
- Includes framing, addressing, and error detection
- Covers connecting nodes on Layers 1 and 2
- Discusses security considerations
- Presents a summary of the chapter content
Chapter 2: Data Link Layer: Representation of Networks as Graphs
- Uses graphs to represent networks
- Includes directed graphs and undirected graphs
- Explains paths in networks and network topologies
- Introduces adjacency and distance matrices
- Explains how to create tree structures
Directed Graphs
- Commonly used to represent network topologies and node connections
- An asymmetric network can be represented as a directed graph G = (N, A)
- N: Set of nodes/vertices
- A: Set of directed arcs {i, j} | i, j ∈ N ∧ i, j are connected by a directed arc}
- Example: N = {1, 2, 3, 4}, A = {(1, 2), (2, 3), (2, 4), (1, 4)}
Undirected Graphs
- Represents a symmetric network as an undirected graph G = (N, E)
- N: Set of nodes
- E: Set of (undirected) edges {i, j} | i, j ∈ N ∧ i, j are connected undirectedly}
- Example: N = {1, 2, 3, 4}, E = {{1, 2}, {2, 3}, {2, 4}, {1, 4}}
Paths in Networks
- A path between two nodes s, t ∈ N is a set Pst = {(s, i), (i, j),...,(k, l),(l, t)} of edges connecting s and t
- Path cost: Sum of costs of used edges: c(Pst) = ΣCij (i,j)∈Pst
- Path length: Number of intermediate nodes: I(Pst) = |Pst|
- Layer 3 path costs are often referred to as hop counts which are less common on Layer 2.
- Source and destination are commonly denoted as 's' and 't' respectively
Network Topologies
- Descriptions of how nodes are interconnected in a network
- Well-known Topologies: Point-to-point, Chain, Star, Mesh, Tree, Bus
Adjacency and Distance Matrix
- Adjacency matrix (A) represents connections between nodes in a network
- A(i, j) = 1 if there's a connection from node i to node j, otherwise 0
- Symmetric if A = AT
- Distance matrix (D) shows the shortest path cost between all node pairs.
- D(i, j) = cost of the shortest path of length 1 between nodes i and j
- D(i, i)= 0, dij= ∞ if no direct connection
Question and Answer About Adjacency and Distance Matrix
- Question: How do we get a matrix of the shortest path costs between any pair of nodes?
- Answer: Calculate powers of D with respect to the min-plus product
- (Dn with d = min {di-1 + dkj} for i, j, k ∈ N)
- The nth power (Dn) contains costs of shortest paths of length n hops or less.
Creating Tree Structures
- Trees are connected, loop-free graphs
- Two special types are discussed: Shortest Path Tree (SPT) and Minimum Spanning Tree (MST)
- SPT connects a root to all other nodes with minimum total cost.
- MST connects all nodes with minimum total cost for the needed arcs (unrooted tree).
Characterizing Connections, Media Access, ALOHA, CSMA, CSMA/CD, CSMA/CA, Token Passing
- Data rate, transmission delay, direction of transmission, and multiple access
- Different media access control methods:
- ALOHA (early wireless networking protocol)
- Slotted ALOHA
- CSMA/CSMA/CD
- CSMA/CA
- Token Passing
Addressing and Error Detection
- Unique node identification
- Broadcast and multicast addresses
- MAC addresses (Media Access Control)
- Error detection codes (e.g., checksums/cyclic redundancy checks (CRC)) are used to increase reliability)
- Handling of frame boundaries and code transparency (essential for reliable transmission and receiver recognition)
Security Considerations (Layer 2)
- CAM (Content Addressable Memory) poisoning, an attack where an attacker can forge MAC addresses and disrupt network communications.
- Denial of Service (DoS) attack is performed by flooding the switch with a large number of requests from arbitrary source addresses
- Mitigations for layer 2 attacks (e.g. Allowed source address, dynamic port security, automatic removal of obsolete entries)
- Cable and wireless security considerations (limiting access to the medium, authentication, wireless security standards, like WEP, WPA, WPA2, and WPA3)
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Description
Test your knowledge on Chapter 2 of Computer Networking and IT Security, focusing on the Data Link Layer. This quiz covers topics such as Local Area Networks (LANs), framing, addressing, error detection, and network representation using graphs. Prepare to demonstrate your understanding of both theoretical concepts and practical applications.
